Heat exchanger

ABSTRACT

A heat exchanger includes a heat transfer tube group in which a plurality of first heat transfer tubes ( 3 ) through which a first fluid flows and a plurality of second heat transfer tubes ( 4 ) through which a second fluid that exchanges heat with the first fluid flows are arranged alternately while being in contact with each other. The heat transfer tube group is formed in a spiral shape by being wound in X direction perpendicular to Y direction in which the first heat transfer tubes ( 3 ) and the second heat transfer tubes ( 4 ) are arranged. A plurality of concave portions ( 3   a ) are provided on both sides, in the X direction, of an outer circumferential surface ( 31 ) of each of the first heat transfer tubes ( 3 ), along an extending direction of the first heat transfer tube ( 3 ). The plurality of concave portions ( 3   a ) form convex portions on an inner circumferential surface of the first heat transfer tube ( 3 ).

TECHNICAL FIELD

The present invention relates to a heat exchanger for exchanging heatbetween a first fluid and a second fluid, particularly to a heatexchanger suitable for heat pump type water heaters.

BACKGROUND ART

In conventional heat pump type water heaters, air conditioners, floorheating devices, etc., a heat exchanger for exchanging heat between twokinds of fluids (water and a refrigerant, or air and a refrigerant, forexample) is used.

For example, Patent Literature 1 discloses a heat exchanger 10 as shownin FIGS. 10A and 10B. In the heat exchanger 10, one circular water tube11 through which water flows and two circular refrigerant tubes 12through which a refrigerant flows are in close contact with each otherover their entire lengths, and these tubes 11 and 12 are formed in atrack-wound shape. The outer diameter of each of the circularrefrigerant tubes 12 is set to be about half of the outer diameter ofthe circular water tube 11. The two circular refrigerant tubes 12 aredisposed at positions at an angle of 45 degrees from the center of thecircular water tube 11 with respect to the horizontal line therebetween.Patent Literature 1 also shows in FIG. 4 a heat exchanger unit in whichthe heat exchangers 10 formed in a track-wound shape are stacked, with aheat insulation sheet being interposed therebetween.

CITATION LIST Patent Literature

PTL 1: JP 2006-162204 A

SUMMARY OF INVENTION Technical Problem

With a configuration in which the circular water tube 11 and thecircular refrigerant tubes 12 are wound while being in contact with eachother as in the heat exchanger 10 disclosed in Patent Literature 1, itis possible to ensure a large length of contact among the tubes in asmall occupation area. Therefore, the heat exchanger 10 can be downsizedmore than other heat exchangers having comparable performances. However,even this type of heat exchanger is required to be downsized further.

Under such circumstances, the present invention is intended to provide aheat exchanger that can be downsized further.

Solution to Problem

The present invention provides a heat exchanger including a heattransfer tube group in which a plurality of first heat transfer tubesthrough which a first fluid flows and a plurality of second heattransfer tubes through which a second fluid that exchanges heat with thefirst fluid flows are arranged alternately while being in contact witheach other. The heat transfer tube group is formed in a spiral shape bybeing wound in a perpendicular direction perpendicular to an arrangementdirection in which the first heat transfer tubes and the second heattransfer tubes are arranged. A plurality of concave portions areprovided on both sides, in the perpendicular direction, of an outercircumferential surface of each of the first heat transfer tubes, alongan extending direction of the first heat transfer tube. The plurality ofconcave portions form convex portions on an inner circumferentialsurface of each first heat transfer tube.

Advantageous Effects of Invention

In the above-mentioned configuration, since both of the first heattransfer tube and the second heat transfer tube constituting thespiral-shaped heat transfer tube group are provided plurally, small-sizetubes can be used as these heat transfer tubes. This makes it possibleto reduce the minimum bend radius of the heat transfer tube group.Moreover, since the first heat transfer tubes and the second heattransfer tubes are arranged in a direction perpendicular to thedirection in which the heat transfer tube group is wound, the width ofthe row of these tubes also can be kept small. Furthermore, since thefirst heat transfer tubes and the second heat transfer tubes arearranged alternately while being in contact with each other, a heattransfer tube of one type is sandwiched between heat transfer tubes ofthe other type, except for the heat transfer tubes located at both sideends. Thus, it is possible to ensure a large contact area between eachfirst heat transfer tube and second heat transfer tube, and accordinglyit is possible to shorten the entire lengths of the first heat transfertube and the second heat transfer tube. With such a configuration, theheat exchanger of the present invention can be downsized furthercompared to conventional heat exchangers having comparable performances.

Furthermore, in the present invention, concave portions are provided onboth sides, in a direction perpendicular to an arrangement direction inwhich the first heat transfer tubes are arranged, of an outercircumferential surface of each of the first heat transfer tubes, alongan extending direction of the first heat transfer tube. The concaveportions form convex portions on an inner circumferential surface ofeach first heat transfer tube. Therefore, the first fluid flows throughthe first heat transfer tube while colliding with the convex portions,so that the flow of the first fluid is disturbed. This makes it possibleto improve the in-plane temperature uniformity of the first fluid andenhance the heat exchanging efficiency between the first fluid and thesecond fluid. As a result, the heat exchanger can be downsized further.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating a heat exchanger according to oneembodiment of the present invention.

FIG. 2 is an enlarged view of an essential part of FIG. 1.

FIG. 3 is an enlarged cross-sectional view of an essential part of FIG.1, taken along the line III-III.

FIG. 4 is an enlarged side view of an essential part of the heatexchanger illustrated in FIG. 1.

FIG. 5A is a cross-sectional view taken along the line VA-VA in FIG. 4.FIG. 5B is a cross-sectional view taken along the line VB-VB in FIG. 4.

FIG. 6A is a graph showing a relationship between the maximum depth ofconcave portions of a second heat transfer tube and the flow velocity ofa refrigerant near an inner circumferential surface of the second heattransfer tube. FIG. 6B is a graph showing a relationship between themaximum depth of the concave portions of the second heat transfer tubeand the pressure loss.

FIG. 7 is an enlarged side view of an essential part of a modified heatexchanger.

FIG. 8 is an enlarged side view of an essential part of another modifiedheat exchanger.

FIG. 9 is a configuration diagram of a heat pump type water heaterincluding the heat exchanger illustrated in FIG. 1.

FIG. 10A is a plan view illustrating a conventional heat exchanger. FIG.10B is a cross-sectional view taken along the line XB-XB in FIG. 10A.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the embodiments for carrying out the present invention willbe described in detail with reference to the drawings. A descriptionwill be made below with respect to, as an example, a heat exchanger forexchanging heat between water and a refrigerant, such as carbon dioxideand chlorofluorocarbon alternative, used for an apparatus such as a heatpump type water heater. However, the present invention is not limited tothis. For example, the present invention is applicable to a heatexchanger for exchanging heat between water and water (hot water), andan internal heat exchanger for exchanging heat between a hightemperature refrigerant and a low temperature refrigerant in a heat pumpcycle.

As illustrated in FIG. 1 to FIG. 3, a heat exchanger 1 according to oneembodiment of the present invention includes a heat transfer tube group2 formed in a spiral shape so as to have a shape of a flat rectangularplate. The heat transfer tube group 2 has a configuration in which aplurality (4 in the example illustrated) of the first heat transfertubes 3 and a plurality (3 in the example illustrated) of the secondheat transfer tubes 4 are joined while being in contact with each otherover the approximately entire lengths and are integrated with eachother. Relatively low temperature water (a first fluid) flows throughthe first heat transfer tubes 3 and a relatively high temperaturerefrigerant (a second fluid) flows through the second heat transfertubes 4, so that the heat is exchanged between the water and therefrigerant and the water is heated by the refrigerant.

The first heat transfer tubes 3 and the second heat transfer tubes 4 maybe made of metal, such as copper, a copper alloy and SUS, having asatisfactory thermal conductivity. Circular tubes are used suitably asthe first heat transfer tubes 3 and the second heat transfer tubes 4.

As shown in FIG. 3, the first heat transfer tubes 3 and the second heattransfer tubes 4 are arranged alternately in a row in a direction (an upand down direction in FIG. 3) perpendicular to extending directions(central axis directions) of the first heat transfer tubes 3 and thesecond heat transfer tubes 4, while being in contact with each other. Inthe present embodiment, the first heat transfer tubes 3 and the secondheat transfer tubes 4 are arranged so that their centers lie on the samestraight line. One first heat transfer tube 3 and one second heattransfer tube 4 adjacent to each other are joined to each other.

The joining between the first heat transfer tube 3 and the second heattransfer tube 4 may be performed by brazing, soldering, use of athermally conductive adhesive, etc. When joined using such a joiningagent, the first heat transfer tube 3 and the second heat transfer tube4 has a large joining area therebetween and can ensure an effective heattransfer area sufficiently. It is also possible to join the first heattransfer tubes 3 and the second heat transfer tubes 4 together bybundling collectively the first heat transfer tubes and the second heattransfer tubes 4 with a heat-shrinkable tube.

Here, the first heat transfer tubes 3 have preferably an outer diameterD₁ equal to or larger than an outer diameter D₂ of the second heattransfer tubes 4 (D₂≦D₁). The first heat transfer tubes 3 in the presentembodiment have an outer diameter and wall thickness larger than thoseof the second heat transfer tubes 4. For example, in the case of usingcarbon dioxide (CO₂) as the refrigerant, the outer diameter D₂ of thesecond heat transfer tubes 4 is 5.0 mm and the outer diameter D₁ of thefirst heat transfer tubes 3 is 6.0 mm.

The heat transfer tube group 2 is wound in a perpendicular direction(hereinafter referred to as “X direction”) perpendicular to anarrangement direction (hereinafter referred to as “Y direction”) inwhich the first heat transfer tubes 3 and the second heat transfer tubes4 are arranged. Specifically, as shown in FIG. 1, the heat transfer tubegroup 2 is formed in an approximately-rectangular spiral shape that iswound while repeating alternately a straight portion 2 a and aquarter-arc bent portion 2 b that smoothly is bent approximately 90°.

Preferably, a gap S (see FIG. 2 and FIG. 3) is formed between anouter-located winding portion and an inner-located winding portionadjacent to each other, that is, between an n-th portion (n is a naturalnumber) and an n+1-th portion when counting from the outside, in theheat transfer tube group 2. The thus formed gap S can prevent directheat transfer between winding portions adjacent to each other in theheat transfer tube group 2. As a spacer, a copper tube or a resin sheet,for example, may be disposed at an appropriate location between theouter-located winding portion and the inner-located winding portionadjacent to each other in the heat transfer tube group 2 in order toensure the gap S. Alternatively, a heat insulating material may beinterposed between the winding portions adjacent to each other. In thiscase, the same advantageous effects also can be obtained as in the caseof forming the gap S.

As shown in FIG. 2, all bend radii R of the bent portions 2 b in theheat transfer tube group 2 preferably are uniform. Such a configurationcan reduce the number of the types of jigs used in the bending process,improving the workability.

In the present embodiment, the water flows through the first heattransfer tubes 3 from a peripheral side toward a central side of thespiral shape of the heat transfer tube group 2, and the refrigerantflows through the second heat transfer tubes 4 from the central sidetoward the peripheral side of the spiral shape of the heat transfer tubegroup 2. Such a configuration allows the water and the refrigerant toform mutually opposed flows, and thereby the heat is exchangedeffectively therebetween.

Specifically, a first outlet member 6 and a second inlet member 7 aredisposed on the central side of the spiral shape of the heat transfertube group 2, and a first inlet member 5 and a second outlet member 8are disposed on the peripheral side of the spiral shape of the heattransfer tube group 2. The members 5 to 8 have a rectangularparallelepiped shape extending in the Y direction, and have internalspaces 51, 61, 71 and 81, respectively, at one end surface in the longerdirection (the end surface illustrated in FIG. 1). One end of each firstheat transfer tube 3 is connected to one side surface of the firstoutlet member 6, and the other end of each first heat transfer tube 3 isconnected to one side surface of the first inlet member 5. One end ofeach second heat transfer tube 4 is connected to one side surface of thesecond inlet member 7, and the other end of each second heat transfertube 4 is connected to one side surface of the second outlet member 8.That is, the first inlet member 5 forms water inlets for guiding waterinto the respective first heat transfer tubes 3, whereas the firstoutlet member 6 forms water outlets for discharging collectively thewater that has flowed through the first heat transfer tubes 3. Thesecond inlet member 7 forms refrigerant inlets for guiding refrigerantinto the respective second heat transfer tubes 4, whereas the secondoutlet member 8 forms refrigerant outlets for discharging collectivelythe refrigerant that has flowed through the second heat transfer tubes4.

Furthermore, in the present embodiment, a plurality of concave portions3 a and 4 a as shown in FIG. 4, FIGS. 5A and 5B are provided in aspecified region E₁ of each longer-side straight portion 2 a and aspecified region E₂ of each shorter-side straight portion 2 a in theheat transfer tube group 2 illustrated in FIG. 1. In this way, theconcave portions 3 a and 4 a preferably are provided avoiding the bentportions 2 b when the bend radii R of the bent portions 2 b are small.Thereby, damages during the bending process can be prevented. Here, thespecified regions E₁ and E₂ each may be across the entire length of thecorresponding straight portion 2 a or may be narrower than this.Alternatively, the lengths of the specified regions E₁ and E₂ maydecrease toward the inner side of the spiral shape. Moreover, theconcave portions 3 a and 4 a do not need to be provided on both of thelonger-side straight portions 2 a and the shorter-side straight portions2 a, and may be provided to either the longer-side straight portions 2 aor the shorter-side straight portions 2 a.

Specifically, in the specified regions E₁ and E₂, the plurality ofconcave portions 3 a are provided on both sides, in the X direction, ofan outer circumferential surface 31 of each of the first heat transfertubes 3 at a specified pitch along the extending direction of the firstheat transfer tube 3. Also, the plurality of concave portions 4 a areprovided on both sides, in the X direction, of an outer circumferentialsurface 41 of each of the second heat transfer tubes 4 at a specifiedpitch along the extending direction of the second heat transfer tube 4.As shown in FIG. 5A, the concave portions 3 a provided on the first heattransfer tube 3 form convex portions 3 b on an inner circumferentialsurface 32 of each first heat transfer tube 3. As shown in FIG. 5B, theconcave portions 4 a provided on the second heat transfer tube 3 formconvex portions 4 b on an inner circumferential surface 42 of eachsecond heat transfer tube 4. The concave portions 3 a need only beprovided on both sides, in the X direction, of the outer circumferentialsurface 31 of each of the heat transfer tubes 3, and the concaveportions 4 a need only be provided on both sides, in the X direction, ofthe outer circumferential surface 41 of each of the heat transfer tubes4, and thus the concave portions 3 a and 4 a do not necessarily have tobe located just lateral to the centers of the heat transfer tubes 3 and4, respectively. For example, in FIG. 4, the concave portions 3 a may beprovided at positions upward or downward off the positions just lateralto the center of the heat transfer tube 3, and the concave portions 4 amay be provided at positions upwardly or downwardly off the positionsjust lateral to the center of the heat transfer tube 4.

In the present embodiment, the concave portions 3 a provided on oneside, in the X direction, of the outer circumferential surface 31 ofeach of the first heat transfer tubes 3 and the concave portions 3 aprovided on the other side, in the X direction, of the outercircumferential surface 31 of each of the first heat transfer tubes 3are disposed alternately along the extending direction of the first heattransfer tube 3. Likewise, the concave portions 4 a provided on oneside, in the X direction, of the outer circumferential surface 41 ofeach of the second heat transfer tubes 4 and the concave portions 4 aprovided on the other side, in the X direction, of the outercircumferential surface 41 of each of the second heat transfer tubes 4are disposed alternately along the extending direction of the secondheat transfer tube 4. Furthermore, in the present embodiment, theconcave portions 3 a provided on each first heat transfer tube 3 and theconcave portions 4 a provided on each second heat transfer tube 4 arelinear recesses extending in a direction parallel to the extendingdirection of the first heat transfer tube 3 or the second heat transfertube 4.

For example, when carbon dioxide is used as the refrigerant, the concaveportions 4 a with a length of 5.0 mm are provided on both sides, in theX direction, of each of the second heat transfer tube 4 at a pitch of 10mm, and the concave portions 3 a with a length of 5.0 mm are provided onboth sides, in the X direction, of each of the first heat transfer tube3 at a pitch of 10 mm. The pitch refers to a center-to-center distancebetween adjacent concave portions on one side in the X direction. Themaximum depths (depths at the lowest points located at the deepestpositions) of the concave portions 3 a and 4 a are 5% or more but 20% orless of the outer diameters of the heat transfer tubes 3 and 4,respectively.

In order to form the heat transfer tube group 2 with such aconfiguration, the first heat transfer tubes 3 and the second heattransfer tubes 4 both of which are straight are stacked alternately,these tubes stacked are joined by the above-mentioned method, and thenthe concave portions 3 a and 4 a are formed on both sides, right andleft, of the heat transfer tube group 2 by pressing, for example.Thereafter, the heat transfer tube group 2 is bent, on the same plane,into an approximately-rectangular spiral shape. Alternatively, it isalso possible to bend individually, on the same plane, the first heattransfer tubes 3 and the second heat transfer tubes on which the concaveportions 3 a and 4 a respectively are formed in advance by pressing,etc. into an approximately-rectangular spiral shape and stack them.

As described above, in the heat exchanger 1 in the present embodiment,since both of the first heat transfer tube 3 and the second heattransfer tube 4 constituting the spiral heat transfer tube group 2 areprovided plurally, small-size tubes can be used as these heat transfertubes. This makes it possible to reduce the minimum bend radius of theheat transfer tube group 2. Moreover, since the first heat transfertubes 3 and the second heat transfer tubes 4 are arranged in a directionperpendicular to the direction in which the heat transfer tube group 2is wound, the width of the row of these tubes also can be kept small.Furthermore, since the first heat transfer tubes 3 and the second heattransfer tubes 4 are arranged alternately while being in contact witheach other, a heat transfer tube of one type is sandwiched between heattransfer tubes of the other type, except for the heat transfer tubeslocated at both side ends. Thus, it is possible to ensure a largecontact area between each first heat transfer tube 3 and second heattransfer tube 4, and accordingly it is possible to shorten the entirelengths of the first heat transfer tube and the second heat transfertube. With such a configuration, the heat exchanger 1 of the presentinvention can be downsized further compared to conventional heatexchangers having comparable performances.

Furthermore, in the heat exchanger 1 in the present embodiment, theconcave portions 3 a are provided on both sides, in the X direction, ofthe outer circumferential surface 31 of each of the first heat transfertubes 3, along the extending direction of the first heat transfer tube3. The concave portions 3 a form the convex portions 3 b on the innercircumferential surface 32 of each first heat transfer tube 3.Therefore, the water flows through the first heat transfer tubes 3 whilecolliding with the convex portions, so that the flow of the water isdisturbed. This makes it possible to improve the in-plane temperatureuniformity of the water and enhance the heat exchanging efficiencybetween the water and the refrigerant. As a result, the heat exchangercan be downsized further. In addition, since the concave portions 3 aare not provided in the Y direction in which the first heat transfertubes 3 and second heat transfer tubes 4 are in contact with each otherbut are provided in the X direction, the above-mentioned effects can beobtained without increasing thermal contact resistance of these tubes.

Moreover, in the present embodiment, the concave portions 4 a also areprovided on both sides, in the X direction, of the outer circumferentialsurface 41 of each of the second heat transfer tubes 4, along theextending direction of the second heat transfer tube 4. The concaveportions 4 a form the convex portions 4 b on the inner circumferentialsurface 42 of each second heat transfer tube 4. Also, the refrigerantflows through the second heat transfer tubes 4 while colliding with theconvex portions 4 b. Accordingly, the flow of the refrigerant isdisturbed as well, so that the heat exchanging efficiency between thewater and the refrigerant is enhanced further. As the second heattransfer tubes 4 for the refrigerant, grooved tubes in each of which aplurality of grooves are provided on the inner circumferential surfacecan be used instead of the circular tubes in each of which the concaveportions 4 a are provided on the outer circumferential surface 41.However, since such grooved tubes are expensive, the cost may be reducedby using, as in the present embodiment, the circular tubes in each ofwhich the concave portions 4 a are provided on the outer circumferentialsurface 41.

As shown in FIGS. 10A and 10B, in the heat exchanger 10 having atrack-wound shape, in other words, including a pair of straight portionsdisposed in parallel so as to face each other and a pair of semicirculararc portions bent 180° so as to connect end portions of these straightportions to each other, a large dead space with a shape of anapproximately right-angled triangle is formed outside each semicirculararc portion, making a factor of increasing the occupancy area. Incontrast, in the heat exchanger 1 in the present embodiment, since theheat transfer tube group 2 is formed in an approximately-rectangularspiral shape and the bent portions 2 b located at corners of the spiralshape have the uniform bend radii R, the bend radius of each bentportion 2 b located in the outermost winding portion is significantlysmaller than that of the track-wound shape. This makes it possible toreduce the dead spaces formed outside the heat exchanger 1. From anotherviewpoint, the configuration in the present embodiment is different fromthe track-wound configuration in that the bend radii of the bentportions 2 b do not decrease from the peripheral side toward the centralside of the spiral shape. Therefore, the heat transfer tube group 2 canreach near the center of the spiral shape, and thus the dead spaces nearthe center can be reduced. Moreover, the uniform bend radii R of thebent portions 2 b lead to satisfactory workability.

Furthermore, since small-size tubes can be used as the first heattransfer tubes 3 and the second heat transfer tubes 4 as describedabove, it is possible to make the bend radii of the bent portions 2 b ofthe spiral-shape heat transfer tube group 2 smaller and reduce furtherthe dead spaces having the shape of an approximately right-angledtriangle that are formed outside the heat exchanger 1 by the bentportions 2 b.

Furthermore, in the present embodiment, the first inlet member 5 and thesecond outlet member 8 are disposed on the peripheral side of the spiralshape of the heat transfer tube group 2, and the first outlet member 6and the second inlet member 7 are disposed on the central side of thespiral shape of the heat transfer tube group 2. In other words, therelatively low temperature water flows through the first heat transfertubes 3 from one end located on the peripheral side of the spiral shapetoward the other end located on the central side of the spiral shape,and the relatively high temperature refrigerant flows through the secondheat transfer tubes 4 from one end located on the central side of thespiral shape toward the other end located on the peripheral side of thespiral shape. That is, when the heat exchanger 1 is observed as a whole,both of the water and the refrigerant flow so that the temperaturesthereof increase from the periphery toward the center of the heatexchanger 1, and thereby the high temperature portion from which a largeamount of heat is radiated to the outside can be disposed in a smallarea and the radiation loss can be suppressed more effectively.Moreover, since the viscosity of water lowers as its temperatureincreases, the configuration in which water flows so that itstemperature increases toward the center of the spiral shape ispreferable also from the viewpoint of pressure loss.

The inwardly-protruding convex portions 4 b of the second heat transfertubes 4 through which the refrigerant flows have the following effects.Usually, the refrigerant contains an oil, such as PAG (polyalkyleneglycol), for lubricating compressors, etc. This causes the flow in eachsecond heat transfer tube 4 to be a two-layer flow, forming an oil filmon the inner circumferential surface 42 of the second heat transfer tube4. In order to maintain a high heat exchanging efficiency, the thicknessof the oil film preferably is as small as possible. The convex portions4 b are effective also in reducing the thickness of the oil film. Morespecifically, the presence of the convex portions 4 b increases the flowvelocity of the refrigerant near the inner circumferential surface 42,thereby increasing the difference between the velocity of the oil filmflowing on the inner circumferential surface 42 and the velocity of therefrigerant. In such a situation, the refrigerant takes away a largeamount of the oil from the surface of the oil film, reducing thethickness of the oil film. On the other hand, when the convex portions 4b have an excessively large height, the pressure loss is increased andthe performance of the heat exchanger 1 is deteriorated. Therefore, itis preferable to set appropriately the maximum depth of the concaveportions 4 a and hold the height of the convex portions 4 b within aproper range.

For example, FIGS. 6A and 6B show the results of analyses on theflowability of the refrigerant, which was carbon dioxide, when themaximum depth of the concave portions 4 a of the second heat transfertube 4 was changed. The analyses were made using a software “FULENT6.3”, under the conditions that the refrigerant had a mass flow rate of650 kg/m²s, a temperature of 60° C. and a pressure of 10 MPa, and theoil concentration in the refrigerant was 1.0 mass %. The concaveportions 4 a with a length of 5.0 mm were provided on both sides, in theX direction, of each of the second heat transfer tubes 4 at a pitch of10 mm, as shown in FIG. 4. The second heat transfer tubes 4 had an outerdiameter of 5.0 mm and an inner diameter of 4.1 mm. Then, a calculationwas made in each of the cases where the maximum depth of the concaveportions 4 a was 0 mm, 0.4 mm, 0.5 mm, and 0.6 mm. 0 mm of the maximumdepth of the concave portions 4 a indicates that circular tubes havingno concave portions 4 a were used.

As shown in FIG. 6A, the flow velocity of the refrigerant near the innercircumferential surface 42 is converged when the maximum depth of theconcave portions 4 a is in the range of 0.4 to 0.5 mm. This means thatthe thickness of the oil film is not reduced even if the maximum depthof the concave portions 4 a is increased to be more than that. On theother hand, as shown in FIG. 6B, the pressure loss is increased rapidlywhen the maximum depth of the concave portions 4 a is in the range of0.4 to 0.5 mm. Therefore, it is preferable that the maximum depth of theconcave portions 4 a is in the range of 0.3 to 0.6 mm, which is slightlywider than the above-mentioned range in two directions.

The above-mentioned heat exchanger 1 is used suitably for a heat pumptype water heater 200. FIG. 9 shows the heat pump type water heater 200including the heat exchanger 1 of the present embodiment. The heat pumptype water heater 200 has a heat pump unit 201 and a tank unit 203. Thetank unit 203 has a hot water reservoir tank 202 for holding the hotwater produced in the heat pump unit 201. The hot water held in the hotwater reservoir tank 202 is supplied to a hot water tap 204. The heatpump unit 201 includes a compressor 205 for compressing the refrigerant,a radiator 207 that allows the refrigerant to radiate heat, an expansionvalve 209 for expanding the refrigerant, an evaporator 211 forevaporating the refrigerant, and a refrigerant tube 213 connecting thesedevices in this order. The heat exchanger 1 in the present embodiment isused as the radiator 207. In the heat pump unit 201, a positivedisplacement expander capable of recovering the expansion energy of therefrigerant may be used instead of the expansion valve 209.

The present invention is not limited to the above-mentioned embodimentand can be modified variously. For example, the number and the outerdiameter of the first heat transfer tubes 3 and the second heat transfertubes 4 can be selected appropriately according to the performancerequired for the heat exchanger 1 and the types of the first fluid andthe second fluid. In addition, the number of windings that the heattransfer tube group 2 makes and the size of its spiral shape also can bedetermined appropriately.

Furthermore, the heat transfer tube group 2 does not need to be formedin an approximately-rectangular spiral shape. For example, it may beformed in a circular spiral shape, or in a track-wound shape as shown inFIG. 10A. However, from the viewpoint of the dead space as mentionedabove, it is preferable that the heat transfer tube group 2 is formed inan approximately-rectangular spiral shape.

In the present embodiment, the first heat transfer tubes 3 and thesecond heat transfer tubes 4 are arranged so that their centers lie onthe same straight line. However, when the outer diameter D₁ of the firstheat transfer tubes 3 is different from the outer diameter D₂ of thesecond heat transfer tubes 4, the first heat transfer tubes 3 and thesecond heat transfer tubes 4 may be arranged so that their outermostpoints on one side in the perpendicular direction perpendicular to thearrangement direction lie on the same straight line, for example. Inthis case, the centers of the first heat transfer tubes 3 and thecenters of the second heat transfer tube 4 lie in a staggered manner.

Although the concave portions 3 a provided on one side, in the Xdirection, of the outer circumferential surface 31 of each of the firstheat transfer tubes 3 and the concave portions 3 a provided on the otherside, in the X direction, of the outer circumferential surface 31 ofeach of the first heat transfer tubes 3 are disposed alternately alongthe extending direction of the first heat transfer tube 3 in theabove-mentioned embodiment, they may be disposed at positions facingeach other in the X direction. However, when the concave portions 3 aare parallel to the extending direction of the first heat transfer tube3, since the concave portions 3 a thus disposed elongate narrow portionsin the first heat transfer tube 3, the concave portions 3 a preferablyare disposed as in the above-mentioned embodiment. This is also the casewith the concave portions 4 a provided on the second heat transfer tubes4.

Furthermore, as shown in FIG. 7, the concave portions 3 a provided onboth sides, in the X direction, of the outer circumferential surface 31of each of the first heat transfer tubes 3 may be linear recessesextending in a direction inclined with respect to the extendingdirection of the first heat transfer tube 3. The concave portions 4 aprovided on both sides, in the X direction, of the outer circumferentialsurface 41 of each of the second heat transfer tubes 4 may be linearrecesses extending in a direction inclined with respect to the extendingdirection of the second heat transfer tube 4. Such concave portions 3 aand 4 a allow the water or the refrigerant to flow while stirring themeffectively. Particularly, in the case where the heat exchanger 1 isused for the heat pump type water heater 200 as shown in FIG. 9, it ispreferable that the concave portions 4 a provided on the second heattransfer tube 4 through which the refrigerant flows are inclined withrespect to the extending direction of the second heat transfer tube 4.In some cases, the refrigerant contains an oil for lubricating thecompressor 205, and a relatively large amount of this oil accumulates onthe bottom of the second heat transfer tube 4, lowering the heatexchanging efficiency. In such a case, the inclined concave portions 4 acould stir the refrigerant and suppress the accumulation of the oil. Inthe case where the inclined concave portions 3 a and 4 a are provided onthe heat transfer tubes 3 and 4, respectively, the concave portions 3 aprovided on one side, in the X direction, of the heat transfer tube 3and the concave portions 3 a provided on the other side, in the Xdirection, of the heat transfer tube 3 may be disposed at positionsfacing each other in the X direction, and the concave portions 4 aprovided on one side, in the X direction, of the heat transfer tube 4and the concave portions 4 a provided on the other side, in the Xdirection, of the heat transfer tube 4 may be disposed at positionsfacing each other in the X direction, as shown in FIG. 7. Alternatively,the concave portions 3 a provided on one side, in the X direction, ofthe heat transfer tube 3 and the concave portions 3 a provided on theother side, in the X direction, of the heat transfer tube 3 may bedisposed alternately along the extending direction of the heat transfertube 3, and the concave portions 4 a provided on one side, in the Xdirection, of the heat transfer tube 4 and the concave portions 4 aprovided on the other side, in the X direction, of the heat transfertube 4 may be disposed alternately along the extending direction of theheat transfer tube 4, as shown in FIG. 8.

Furthermore, the shapes and positions of the concave portions 3 a and 4a also can be selected appropriately in combination such that the firstheat transfer tube 3 is provided with the concave portions 3 a parallelto the extending direction whereas the second heat transfer tube 4 isprovided with the concave portions 4 a inclined with respect to theextending direction, and that the concave portions 3 a provided on bothsides of the first heat transfer tube 3 are disposed alternately whereasthe concave portions 4 a provided on both sides of the second heattransfer tube 4 are disposed at the positions facing each other.

The concave portions of the present invention do not need to be linearrecesses as long as they form convex portions on the innercircumferential surface of each first heat transfer tube or second heattransfer tube. For example, the first heat transfer tube 3 and thesecond heat transfer tube 4 may be formed in a wave shape meandering inthe X direction so that valley portions of the wave shape may serve asthe concave portions. That is, the convex portions of the presentinvention do not need to reduce the cross-sectional area of a spaceenclosed by the inner circumferential surface of the first heat transfertube or the second heat transfer tube. The convex portions may beportions protruding inwardly while maintaining the cross-sectional area.However, from the viewpoint of workability, it is preferable that theconcave portions of the present invention are recesses, particularlylinear recesses extending in a specified direction, forming the convexportions 3 b that reduce the cross-sectional area of a space enclosed bythe inner circumferential surface of the first heat transfer tube 3 orthe second heat transfer tube 4, as in the above-mentioned embodiments.

INDUSTRIAL APPLICABILITY

The heat exchanger of the present invention is useful as a heatexchanger for a heat pump, particularly as a heat exchanger for a heatpump type water heater. In addition, the present invention is applicableto a heat exchanger for exchanging heat between liquids or betweengases.

1. A heat exchanger comprising a heat transfer tube group in which aplurality of first heat transfer tubes through which a first fluid flowsand a plurality of second heat transfer tubes through which a secondfluid that exchanges heat with the first fluid flows are arrangedalternately while being in contact with each other, the heat transfertube group being formed in a spiral shape so as to have a shape of aflat rectangular plate by being wound in a perpendicular directionperpendicular to an arrangement direction in which the first heattransfer tubes and the second heat transfer tubes are arranged, whereina plurality of concave portions are provided on both sides, in theperpendicular direction in which the first heat transfer tubes and thesecond heat transfer tubes are out of contact with each other, of anouter circumferential surface of each of the first heat transfer tubes,along an extending direction of the first heat transfer tube, theplurality of concave portions form convex portions on an innercircumferential surface of the first heat transfer tube.
 2. The heatexchanger according to claim 1, wherein the concave portions provided onone side, in the perpendicular direction, of the outer circumferentialsurface of the first heat transfer tube and the concave portionsprovided on the other side, in the perpendicular direction, of the outercircumferential surface of the first heat transfer tube are disposedalternately along the extending direction of the first heat transfertube.
 3. The heat exchanger according to claim 1, wherein the concaveportions provided on one side, in the perpendicular direction, of theouter circumferential surface of the first heat transfer tube and theconcave portions provided on the other side, in the perpendiculardirection, of the outer circumferential surface of the first heattransfer tube are disposed at positions facing each other in theperpendicular direction.
 4. The heat exchanger according to claim 1,wherein a plurality of concave portions also are provided on both sides,in the perpendicular direction, of an outer circumferential surface ofeach of the second heat transfer tubes, along an extending direction ofthe second heat transfer tube, and the plurality of concave portionsform convex portions on an inner circumferential surface of the secondheat transfer tube.
 5. The heat exchanger according to claim 4, whereinthe concave portions provided on one side, in the perpendiculardirection, of the outer circumferential surface of the second heattransfer tube and the concave portions provided on the other side, inthe perpendicular direction, of the outer circumferential surface of thesecond heat transfer tube are disposed alternately along the extendingdirection of the second heat transfer tube.
 6. The heat exchangeraccording to claim 4, wherein the concave portions provided on one side,in the perpendicular direction, of the outer circumferential surface ofthe second heat transfer tube and the concave portions provided on theother side, in the perpendicular direction, of the outer circumferentialsurface of the second heat transfer tube are disposed at positionsfacing each other in the perpendicular direction.
 7. The heat exchangeraccording to claim 1, wherein the concave portions are linear recessesextending in a specified direction.
 8. The heat exchanger according toclaim 7, wherein the specified direction is a direction parallel to theextending direction of the first heat transfer tube.
 9. The heatexchanger according to claim 7, wherein the specified direction is adirection inclined with respect to the extending direction of the firstheat transfer tube.
 10. The heat exchanger according to claim 1, whereinthe heat transfer tube group is formed in an approximately-rectangularspiral shape that is wound while repeating alternately a straightportion and a bent portion that is bent approximately 90° C. with auniform bend radius.
 11. The heat exchanger according to claim 1,wherein a gap is formed between an outer-located winding portion and aninner-located winding portion adjacent to each other in the heattransfer tube group.
 12. The heat exchanger according to claim 1,wherein a heat insulating material is interposed between anouter-located winding portion and an inner-located winding portionadjacent to each other in the heat transfer tube group.
 13. The heatexchanger according to claim 1, wherein the first fluid is heated by thesecond fluid.
 14. The heat exchanger according to claim 13, wherein thefirst fluid is water and the second fluid is a refrigerant.
 15. The heatexchanger according to claim 1, wherein both of the first heat transfertubes and the second heat transfer tubes are circular tubes, and thefirst heat transfer tubes have an outer diameter equal to or larger thanthat of the second heat transfer tubes.
 16. The heat exchanger accordingto claim 1, wherein the first fluid flows through the first heattransfer tubes from an peripheral side toward a central side of thespiral shape, and the second fluid flows through the second heattransfer tubes from the central side toward the peripheral side of thespiral shape.
 17. The heat exchanger according to claim 4, wherein theconcave portions are linear recesses extending in a specified direction.18. The heat exchanger according to claim 17, wherein the specifieddirection is a direction parallel to the extending direction of thesecond heat transfer tube.
 19. The heat exchanger according to claim 17,wherein the specified direction is a direction inclined with respect tothe extending direction of the second heat transfer tube.